The field of this invention is felts for use in papermaking machinery, and more particularly, press felts for use in the press section of a papermaking machine.
Generally, press felts are used in papermaking machines to support the moist, freshly formed paper web as it passes through a variety of rolls which extract water from the moist paper web. In addition to supporting the paper web, the press felt receives the water removed from the paper sheet. Thus, the felt should contain voids to hold the water squeezed from the web. The press felt is normally joined into an endless conveyor belt-like shape. During the various operations previously described, a large amount of water builds up in the press felt. The water is removed by suction or various other drainage devices, usually after the paper web and press felt have separated.
The press felt serves several functions and thus, must simultaneously satisfy conflicting requirements. First, it supports the paper web. In order not to unduly mark the paper, the fabric must present a relatively fine weave or fine grain structure to the paper web. Second, the felt must absorb the water pressed out of the paper. Thus it must be absorbent and relatively open woven. Additionally, the felt drives the press rolls. Thus, it must be relatively durable and longitudinally inextensible.
Because the felt serves several functions, multilayer felts have been designed with a fine structured paper-contacting layer (a batt) which is adhered to a relatively incompressible base fabric. The base fabric does not contact the paper web. Other layers, for instance a compressible, absorbant layer, may be sandwiched between the paper-contacting layer and the base fabric.
Features of the felt may also aid in minimizing vibration problems related to the pressing of water from the web. The press section of a papermaking machine includes transport rollers that move the papermaking fabric along, with the paper web supported above the fabric. Above the paper web are arranged several press rolls. An upper press roll is located opposite a cooperating roll located below the papermaking fabric. The purpose of the press roll is to press down upon the wet web and squeeze water out from the paper and into the fabric. The rolls press down with a force of about 700 pounds per lineal inch. Thus, for a roll forty feet wide, the roll experiences 336,000 pounds of force. At the same time, the paper moves past the roll at 1,000 meters per minute. Thus, the press roll is spinning at a high rate, under an extreme load. The roll is supported from above by a frame, with shock absorber type elements interposed between the roll and the frame. However, any slight imbalance in the roll, which invariably arises, causes the press roll to shake and vibrate. Due to the extreme forces, this vibration causes flattening of the roll, which further exacerbates the vibration. Eventually, the roll vibrates to such a degree that the entire portion of the building housing the press section quakes.
Aside from the obvious noise pollution and physical discomfort, a wildly vibrating roll presents several other problems. First, paper manufactured with such a roll is irregular and cannot be used for fine applications. Further the vibration of the roll ruins the rolls, which must be removed and reground to return to a balanced state. Similarly, the vibration loosens many of the components in the papermaking machine itself. The machine then must be frequently serviced. Finally, the vibration causes the papermaking fabric to compact to a certain degree, such that it no longer exhibits the desired drainage and vibration accomodation characteristics.
Mechanical means, such as frequent balancing of the roll and sophisticated shock absorbing systems have been used to minimize the vibration problem.
An attempted solution to overcome this vibration problem has been to modify the felt. According to one method, disclosed in a European Patent No. 0059973, to Dufour, a papermakers' fabric is disclosed having special yarns. The special yarns are each made up of a multiplicity of small diameter fibers, knitted, knit-braided or otherwise stitched to form a substantially hollow, compressible structure. This knitted yarn is woven into a papermaking fabric as a major constituent of the fabric. The fabric may be multi-layer, having at least one layer comprising a large amount of the special yarns. Because the multifilament yarns are compressible and substantially hollow in the relaxed state, they cushion vibration by compressing. A drawback of this special collapsible yarn fabric is that the yarns are relatively complicated to manufacture. Further, this knitbraided structure loses its resilience relatively quickly.
Thus, it is an object of this invention to provide a papermaking fabric for the press section of a papermakers' machine that minimizes the amplitude of the vibration of the press roll.
It is a further object of the present invention to provide such a papermakers' fabric that is relatively inexpensive and easy to manufacture.
Finally, it is an object of the present invention to provide such a fabric that is relatively light weight and easy to install.